High temperature, typically endothermic, reactions such as steam methane reforming to form hydrogen from steam and hydrocarbons, and pyrolysis of hydrocarbons to produce olefins are typically carried out in furnace tubes with direct fired radiant heat transfer to the outside surface of the tubes, and flowing reactants on the inside of the tubes. Direct fired heat is often beneficial because of the high temperature level of heat required, high heat flux required, and relatively low capital cost of the furnace. But it is difficult to maintain uniform heat transfer in a fired furnace. Therefore, these furnace tube must be operated at an average tube skin temperature that is somewhat below the maximum allowable tube skin temperature because of variations in the tube skin temperatures. This variation is compounded by difficulty in measuring these temperatures. Controlling maximum temperatures is important because coke will generally develop more rapidly on the tube-side at hotter spots. Thicker coke imparts increased resistance to heat transfer, and causes the hot spot to become hotter. This effect snow-balls, and can result in failure of the tube if not detected and corrective action taken. The corrective action is typically to decrease firing of one or more burners in the vicinity of the hot spot. Decreasing firing then lowers heat transfer around the hot spot, and generally is detrimental to performance of the heater.
Lengths of tubes in fired furnaces are also generally limited due to physical constraints. In some steam-methane reforming furnaces multiple levels of burners are provided in order to distribute radiant heat more evenly to the tubes, but even with multiple levels of burners, the vertical distance over which burners can be provided is limited because of the difficulty of providing fuel and air distribution with varying amounts of draft in the furnace. Thus, when a long flowpath within a furnace is desired, multiple passes are generally provided with a plurality of elbows within the firebox. These elbows are common points of problems due to uneven flow and temperatures, and possible erosion along the inside radius.
Combustion of fuels to provide heat inherently generates nitrogen oxides (“NOx”) as a result of exposure of nitrogen, oxygen and free-radicals at elevated temperatures. In certain areas, emission of NOx is limited, and expensive measures such as flue gas treatments such as Selective Catalytic Reduction DeNox systems are occasionally required. Burner systems are available which reduce generation of NOx by controlling combustion temperatures, but the combustion temperatures are difficult to control, and even under ideal conditions, a significant amount of NOx is generated.
Another problem with typically fired process heaters is the limited efficiency of the radiant section of the heater. Particularly if combustion air preheating is not provided, a considerably amount of the fuel burned is utilized to heat the combustion air to flame temperatures. Even when combustion air preheat is provided, the combustion air preheat typically does not bring the temperature of the combustion air to near flame temperatures. Thus the radiant section efficiencies could be considerably improved with a more effective preheating of combustion air, and preheating of the fuel is typically not practiced because significant preheating can result in coke formation from the fuel.
Many methods have been suggested to cope with direct firing of reaction furnaces. Additives for feeds to pyrolysis furnaces have been proposed, including U.S. Pat. Nos. 5,567,305, and 5,330,970. These component are said to reduce and delay the onset of coke formation, but do not eliminate the formation of coke.
Ceramic coatings and pretreatments to furnace tube have also been suggested as being effective to reduce coking in, for example, U.S. Pat. Nos. 5,600,051, 5,463,159, 5,446,229, and 5,424,095. But like treatments to the feedstocks, they are only marginally effective.
Indirect heating and electrical heating has also been suggested, in for example, U.S. Pat. Nos. 5,559,510, 5,554,347, 5,536,488, 5,321,191, and 5,306,481 as methods to provide more even heat flux into such a reaction. These methods avoid the disadvantages of fired furnaces, but incur additional capital and/or operating costs when compared to fired furnace heaters.
Generally, yields of such reactions as reforming of hydrocarbons to produce hydrogen and carbon oxides, olefin production by pyrolysis of hydrocarbons, and styrene production are improved with increasing temperatures. It is therefore generally desirable to operate at such increased temperatures. These temperatures are generally limited by metallurgical limitations of materials that are economical and consistency of the heat transfer to the tubes.
Further, flameless oxidation as a source of heat is known from U.S. Pat. No. 5,255,742.
It would be desirable to provide a process heater in which the metallurgical limitations could be more closely approached. It would be further desirable to provide such a heater which would not require excessive capital or operating costs, and which operates at a greater thermal efficiency. It would also be desirable to provide a process heater wherein generation of NOx is greatly reduced. It would also be desirable to provide such a process heater wherein the heat can be provided to the process in a controllable fashion. Objects of the present invention therefore include accomplishing these results, and other objects which will become apparent upon examination of the following description of the invention.